This invention relates to positive feed power drills, having a spindle drive gear train and a spindle feed gear train arranged to be driven by the same motor, and having a hydraulic (air) piston to disengage the spindle feed gear train for reversal of the positive feed drive, and more particularly to an interchangeable valve system for actuating the hydraulic system when the spindle has been driven a predetermined amount through a work piece, or when the spindle stops advancing a countersinking bit into a work piece.
Positive feed power drills normally have a single motor for turning a spindle through a drive gear train. The spindle is threaded into a feed gear that turns at a predetermined rate faster than the turning rate of the spindle for advancing the spindle as the drill progresses through a work piece. The feed gear is driven by a gear train from the same motor as the drive gear train. The gear ratio of the drive gear train is selected to be slightly less than the gear ratio of the feed gear train so the feed gear will turn slightly faster than the drive gear. In that way, the spindle is advanced a predetermined amount for each turn. Once the spindle has been advanced sufficiently, a hydraulic piston is actuated to disengage the spindle feed gear train from the motor and lock it in place. As the motor continues to drive the spindle in the same direction, the spindle threads turn inside the locked feed gear to rapidly retract the spindle.
It is often desirable to use the same power drill to countersink a hole that has just been drilled in a workpiece. Any mechanism used to actuate the piston to reverse the positive feed drive after the spindle has been advanced a predetermined extent is not suitable for retraction after countersinking because, when countersinking, the spindle is intended to advance until it cannot progress further. Progress is stopped by a flange on the countersinking bit which engages the top of the workpiece. At that time, the piston should be actuated to reverse the positive feed drive.